I have noticed that kernel updates are breaking my system configuration. If I have grub configured to boot a Xen kernel, and install an updated "conventional" kernel RPM, my grub default gets whacked to point to the new kernel. Unfortunately, this could have a negative effect. A machine that is running multiple logical machines before the upgrade stops running them once it is rebooted. My sense is that the default grub configuration should only be moved forward if the new kernel is of the same "type" as the pre-update default. That is, installing kernel-smp should only update my grub default if the current default is an SMP kernel. Installing a xen kernel should only update my default if the current default is a xen kernel, and so forth. But the important thing is that logical machines shouldn't spontaneously fail to restart merely because the xen kernel update is trailing the conventional kernel update.
Sounds like functionality that needs adding to grubby (which for some reason is in the mkinitrd package).
Sorry about the misdirected package. Grubby sounds plausible as the culprit.
This logic should be handled correctly now, although there's the "extra" piece of needing to set DEFAULTKERNEL in /etc/sysconfig/kernel